1. The Field of the Invention
The present invention relates to field of getter materials. In particular, the present invention relates to processes for making non-evaporable getter materials having a very high porosity and to the getter materials thus obtained.
2. The Relevant Art
Non-evaporable getter materials (known in the art as "NEGs") are widely used for creating and maintaining high vacuum conditions. Such conditions are required commonly for devices such as thermal insulators, lamps and in semiconductor processing chambers. These materials also are used for the purification of gases for application in processes requiring gases of high purity such as semiconductor manufacturing processes. Common NEG materials include metals such as zirconium (Zr), titanium (Ti), niobium (Nb), tantalum (Ta), vanadium (V) and their alloys. The alloys can include additional elements, such as aluminum (Al) and/or iron (Fe), for example, the alloy having a weight percentage composition Zr 84%--Al 16% which is manufactured and sold by SAES.RTM. Getters S.p.A. (Lainate, Italy) under the tradename St 101 or the alloy having a weight percentage composition Zr 70%--V 24.6%--Fe 5.4%, also manufactured and sold by SAES.RTM. Getters under the tradename St 707.
Getter materials act by chemisorption of gases such as carbon monoxide (CO), carbon dioxide (CO.sub.2), water (H.sub.2 O), molecular oxygen (O.sub.2), and molecular hydrogen (H.sub.2). Apart from H.sub.2, which dissociates and diffuses inside the getter material even at low temperatures, the other gases remain chemisorbed on the surface of the getter material over temperatures which range from about 200.degree. C. to about 500.degree. C., depending on the NEG material. The diffusion of the chemisorbed species into the material occurs at higher temperatures.
The surface characteristics of the NEG material play a fundamental role in the sorption of reactive gases. A large specific surface (surface per unit weight) of the material and access of the gases to the surface of the NEG material are parameters of fundamental importance to the performance of the NEG. These parameters would be optimized by the use of NEG materials in the form of powders, but powdered NEG materials cannot be used in practice. Rather, the powder NEG materials either are compressed into pellets which are sintered to impart mechanical strength, loaded and compressed in open containers, or rolled onto a support. Regardless of the form employed, the compression and/or heat sintering operations reduce the specific surface of the NEG materials with respect to the starting powder. Moreover, most of the getter particles reside within the interior bulk of the sintered getter material where the gases to be sorbed have only limited access with a consequential decrease in the sorption capacity of the device and in the gas sorption rate.
German Patent Application DE-A-2,204,714 discloses a method for preparing porous NEG devices based on metallic zirconium. According to this method, graphite powder is added to the zirconium powder, along with a third organic component, for example ammonium carbamate, whose weight may reach the total weights of the zirconium and graphite. During the heat sintering treatment the organic component evaporates, leaving a porous structure consisting of zirconium and graphite which acts as an anti-sintering agent for zirconium; thus preventing an excessive reduction in the specific surface of the zirconium.
However, the above-cited German patent application refers only to the use of elemental components and does not mention the use of alloys. Also, the organic component is added in the form of powder of millimeter-sized grains. Due to the large grain size of the organic component, the final getter device has a high porosity with respect to its geometric volume. The porosity distribution obtained, however, does not enhance access of gases to the surface of the internal NEG material grains. Furthermore, the materials thus prepared have poor mechanical properties.
British Patent GB-2,077,487 discloses a porous NEG material obtained from a powder mixture of a metallic getter material, such as titanium or zirconium, and the previously mentioned St 707 alloy as an anti-sintering agent. According to the disclosure, the particles of metallic material have a size of about 125 .mu.m, while the particles of the St 707 alloy have a size of less than 400 .mu.m but are larger than the size of the metallic component. The patent specification states that the ratio of the sizes of the two components is selected so as to prevent an excessive sintering of the metal during the heat treatment, which would lead to a reduction of the specific surface and consequently to a lower efficiency of the resulting getter device. The use of an organic component is not discussed.
Finally, U.S. Pat. No. 4,428,856 discloses a porous NEG material containing from 50% to 98% titanium by weight, from 1.5% to 30% of a high melting point metal selected from among niobium, tantalum, molybdenum (Mo) and tungsten (W), and from 0.5% to 20% of titanium hydride (TiH.sub.2). This patent states that zirconium powders are readily flammable and explosive, whereby one of the objects of the patent is to provide a getter composition that avoids the use of zirconium.
The porosity and specific surface characteristics of the above-described porous NEG materials, though improved with respect to the conventional NEGs, still are not sufficient for particular applications, such as small-volume getter pumps where high performance is required of the getter material. Thus, it would be advantageous to provide porous NEGs that have good mechanical strength and improved porosity and specific surface characteristics.